The present invention relates generally to light sensing apparatus which are suitable for large-area integration, and more particularly to a light sensing circuit or array of light sensing circuits able to indicate the incidence of light occurring at any time during a polling cycle, and configured to allow structuring of the circuit to provide gain.
Present sensing apparatus, such as solid state sensor arrays or imaging devices, often involve as many as 1 million picture elements ("pixels") or more. These sensing apparatus may be stand-alone, or may be incorporated into a combined sensing/display apparatus such as that disclosed and described in copending U.S. application Ser. No. 07/626,795. Such sensing apparatus are employed in systems such as medical or industrial imaging systems, position or motion sensors, interactive displays, etc.
Typically, the sensing apparatus of these systems is divided up into unique cells capable of individually sensing light incident upon them. Each of these light sensing cells usually includes selected circuitry and a photosensitive element whose current-voltage characteristics change in response to the incidence of light. Knowledge of the correlation between the intensity of light incident on the photosensitive element and the change in current-voltage characteristics allows such a device not only to indicate the incidence of light, but also to indicate the intensity (number of photons) of the incident light. This also facilitates design of a sensing apparatus capable of discriminating between light not intended to be sensed, such as ambient light, and light intended to be sensed, such as light from a light pen. In these light sensing cells, the photosensitive element is typically connected such that an electrical potential is established across it. Light incident on the photosensitive element will result in a detectable change of the potential and/or result in a current flow between two points in the cell. In general, each cell may be checked for the results of incident light. The checking of an individual cell shall be referred to hereafter as "polling" of that cell, and one complete check of all such cells of an array shall be referred to as a "polling cycle".
The arrangement of these cells presents a number of drawbacks and disadvantages. In some present devices, in order for the incident light to be detected, the light must be incident on the photosensitive element at the time it is polled. That is, the incidence of light and the measuring for the effect that it has on a sensor must coincide. If it so happens that light is incident on the photosensitive element only between successive pollings of that element, and not at the time the photosensitive element is polled, the polling will fail to indicate the incidence of light.
In small arrays of light sensing cells, there is a certain amount of tolerance for this coincidence requirement. The time period between successive pollings of a cell is usually smaller than the time period the light is incident on the photosensitive element of that cell. Furthermore, together, the array of cells will detect enough incident light to make up for any such coincidence errors at individual cells. However, when the aforementioned large number of pixels, i.e., 1 million or more, is reached, the time between successive pollings of a cell may increase and the coincidence of the light falling upon the photosensitive element and polling of the cell becomes a real concern. Also, where accuracy of detection is critical, it is desirable to overcome this need for coincidence.
Another drawback or disadvantage of some present light sensing cells, and in particular, arrays of such cells, is the need for three or more separate metal line or interconnections per cell. A common or global power line and a data line tied low or to ground are used to establish the potential across the photosensitive element, and a third line is used to select the cell for polling. It is highly desirable to minimize the number of required interconnections in order to minimize the number of faulty rows of cells per array, minimize the additional required metal per cell, and minimize the size of each cell.
A third drawback or disadvantage of present light sensing cells is that since the photosensitive element forms part of the path across which voltage or current is measured, they are not compatible with circuitry capable of providing gain. A photosensitive element capable of providing gain, particularly photoconductive gain, is described in U.S. Pat. No. 5,083,175, entitled "Method of Using Offset Gated Gap-Cell Thin Film Device As A Photosensor" which is incorporated by reference herein. Use of these elements is desirable since they allow a large dynamic range (i.e., the ratio of photocurrent to dark current) between ON and OFF states and hence a large number of discernible gray levels, and allow switching between photoconductive states with lower levels of light.
In addition to the above, there is also a need for improved large area sensor apparatus in general. Specifically, silicon is a desirable material from which to manufacture sensing apparatus due to its electrical photodependence (e.g., variable conductance as a function of incident light). However, crystalline silicon devices are limited in size by the size of the silicon wafer on which the individual sensors are formed. Amorphous silicon has proved to be an advantageous material from which to form large area arrays due to its attractive electronic characteristics, relatively simple, inexpensive, and low temperature fabrication requirements, and the ability to form large arrays (in excess of 12 inches by 12 inches) on substrates such as glass.